Loans and allowance for credit losses - Individually Assessed Allowance (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 183,973	$ 113,129	$ 31,139	$ 31,139	$ 31,464	$ 30,138	$ 28,932
Individually assessed allowance for credit loss	2,291
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	8,609	8,878		4,805	5,170	4,923	5,348
Individually assessed allowance for credit loss	668
Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	63,629	35,599		10,194	9,842	9,655	9,729
Individually assessed allowance for credit loss	0
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	18,298	12,463		3,112	3,331	3,288	3,428
Individually assessed allowance for credit loss	0
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	11,455	6,811		752	727	755	811
Individually assessed allowance for credit loss	9
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	7,191	4,499		544	574	617	566
Individually assessed allowance for credit loss	0
Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	6,705	7,420		4,109	3,709	3,512	3,132
Individually assessed allowance for credit loss	34
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	57,640	30,444		4,621	4,927	4,478	4,149
Individually assessed allowance for credit loss	1,544
Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	10,446	$ 7,015		$ 3,002	$ 3,184	$ 2,910	$ 1,769
Individually assessed allowance for credit loss	36
Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	12,644
Real Estate | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Real Estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,817
Real Estate | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	190
Real Estate | Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,195
Real Estate | Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Real Estate | Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	928
Real Estate | Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	8,181
Real Estate | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	333
Financial Assets and Equipment
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	4,697
Financial Assets and Equipment | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	4,697
Financial Assets and Equipment | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 0